This invention relates to crank drive systems connecting an engine crank and a motor generator for starting the engine and generating electrical power for starting and other vehicle purposes.
To improve fuel economy, a vehicle engine can be stopped when the vehicle is stopped. An otherwise conventional engine can be modified to perform engine stop start (ESS) functions through a belt-driven motor generator (MG) system connected in the accessory drive. A single MG unit performs the functions of the starter motor and the alternator (generator). Since the belt drive and the MG are used to start the engine, the belt system must be designed for extra robustness against slip to maintain acceptable belt noise levels. A heavy duty multi-ribbed belt, similar in appearance to conventional accessory belts, may be used.
Two important parameters governing belt traction on the pulley grooves are belt wrap and belt tension. Since belt traction is a highly non-linear function of wrap angle, the wrap angle should be maximized within reason, such that excessive belt tensions are not required to prevent slip. The nominal belt tension needs to be optimized, balancing design factors such as belt slip, bearing loads, belt life and component packaging. In addition, the belt tension must be controlled to be consistent during engine speed transients and, in particular, changes in the belt loading from cranking to driving functions.
Conventional belt drive systems in vehicles not having ESS functions typically mount a single belt tensioner on the slack side of the crank pulley and generator. This serves to reduce the belt vibration, caused by crankshaft pulsations, between the crank pulley and the generator pulley. Belt driven ESS systems have been developed which incorporate a single tensioner on the tight side of the generator during generating. This is done so that during an engine start, the span between the MG and the crank is supported by a rigidly-mounted idler pulley.
If a single tensioner designed for proper tension and belt takeup were placed in that location, the tensioner would deflect excessively or abruptly bottom out in travel. Thus, the single tensioner placed on the tight side of the generator must necessarily over tension the belt to allow noise-free and slip-free operation during generating. Also, to reduce the belt vibration noise, very short spans may be necessary, which reduce the belt system robustness (for noise and durability) to small assembly misalignments. Another disadvantage of the single tensioner system is that the generating torque, or regenerative braking torque, is eliminated by belt slip. Thus, if the generating torque is large enough, all belt tension could be lost in the span between the crankshaft, driving pulley, and the MG-driven pulley.
Systems with dual arm tensioners have been developed but, due to packaging constraints, belt wrap may be sacrificed. Thus, to ensure slip-free operation, belt tension must be increased, which in turn reduces belt and bearing life. Therefore, a system was desired that can maximize belt wrap around the crank and MG pulleys while at the same time allowing optimal tensioning for both drive (generating) and cranking (motoring) functions and to dynamically follow the belt loads during transition from driving to cranking.
The present invention provides crank drive belt systems featuring a triple pulley tensioner (TPT) which may be applied in various ways to provide proper tensioning on the belt spans adjacent the motor generator for both starting and generating functions and which provide adequate belt wrap and damping functions for the systems. In a simplified embodiment, the belt system connects a crankshaft or crank pulley with a motor generator pulley and at least one accessory drive pulley such as an air conditioner compressor. The belt spans are tensioned by a triple pulley tensioner which includes a fixed tensioner housing mounting a fixed rotatable pulley and two tensioner arms carrying pulleys for tensioning the belt spans on either side of the motor generator. The fixed pulley is positioned on the third belt span between the accessory and the crank and combines with the tensioner pulleys to maximize belt wrap around the crank pulley, the MG pulley and the accessory pulley.
Preferably, the tensioner of the belt span between the crank and the MG pulley is designed to reach kinematic singularity when the MG is cranking the engine and the belt load is greatest between the MG and the crank pulley. Singularity means that the tensioner arm, or the belt, moves to a position where the force of the belt does not move the tensioner arm further toward a xe2x80x9cbottomed outxe2x80x9d position. Optionally, the second tensioner unit may also be positioned to reach kinematic singularity when the MG is driven by the crank. However, this is less important since the generator driving belt loads are lower than the cranking belt loads.
The tensioner body may contain a single spring for tensioning both arms or separate springs for providing different tensioning loads on the two arms. The fixed tensioner body may mount a surrounding bearing for carrying the idler pulley. The total system should be designed to provide maximum belt wrap around the pulleys where that is important, as in the MG pulley and, secondarily, the crank pulley, while lesser belt wrap on the one or more accessory drive units is generally satisfactory. Since movement of the primary tensioner between the crank and the MG may be limited by the desire for kinematic singularity, the secondary tensioner is necessarily designed to absorb the additional slack created in the system by its design and the need to accommodate belt wear during use.
Many possible arrangements of the elements may be used, as dictated in part by the necessary arrangement of components on the engine and the number of accessories to be driven by the belt system. Therefore, the various embodiments subsequently described are intended to be exemplary only and not to limit the scope of the invention.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.